Generally, a turbine engine has a compressor, a combustor, and a turbine or turbine airfoil. The compressor is upstream from the combustor and is configured to pressurize fluids, such as gases or air, for the combustor. The combustor can have a combustion chamber where fuel and the pressurized fluid are combined and combusted. The turbine is downstream from the combustor, extracts energy from the combustor, and is used to drive the compressor. One or more turbine blades of a turbine are turned by hot, combusted gas generated by the combustor, thereby driving the turbine engine.
As technology advances, turbine engine designers have endeavored to increase combustor exit temperatures and high-pressure turbine stage inlet temperatures to achieve improved efficiency and reduce fuel consumption. However, these increased temperatures can jeopardize the integrity of turbine components, such as the turbine blades. Since turbine performance corresponds to cooling of external surfaces of a turbine, such as a surface on a high-pressure side of a turbine blade, it is generally desirable to provide uniform cooling thereto. Accordingly, to mitigate failure of turbine blades resulting from excessive operating temperatures, film cooling may be incorporated into turbine blade designs.
In film cooling, cool air is bled from the compressor, ducted to one or more internal chambers of the turbine blades, and discharged via one or more cooling channels to form one or more cooling jets. For example, a cooling channel can be a hole which couples an internal chamber of a turbine blade to a surface of the turbine blade. To this end, cool air or gas which is cooler than a free stream can be passed from the compressor to an internal chamber of a turbine blade, to an external surface of the turbine blade, and take form as a cooling jet. As a result of the cooling jets, convective heat transfer to the surface of the turbine blade can be reduced. Cooling channels can have a round cross-section, and be oriented at an angle to an external surface of the turbine blade. These cooling jets can be configured to provide a thin, cool, insulating, boundary layer along the external surface of the turbine blade.
However, film cooling may not be effective when a cooling jet detaches, lifts off, or does not adhere to an external surface of a turbine blade. For example, at momentum ratios above about 0.5, a counter-rotating vortex pair, such as a kidney vortex, is often formed. This counter-rotating vortex pair can cause the cooling jet to separate or lift-off from the surface at a sufficiently high blowing ratio. When lift-off occurs, the cooling jet is lifted away from the surface of the turbine blade, thereby reducing film cooling effectiveness.